This invention relates to thin film photovoltaic devices that utilize Cd rich Hg.sub.1-x Cd.sub.x Te as a variable bandgap material, and to the cathodic electrodeposition of Hg.sub.1-x Cd.sub.x Te thin films with controlled stoichiometry (1-x) and thus with controlled electronic and optical properties.
General electrodeposition procedures for CdTe have been given in U.S. Pat. No. 4,400,244 granted to F. A. Kroger, R. L. Rod and M. P. R. Panicker, and assigned to Monosolar, Inc. Briefly, to form a cadmium telluride coating on a conductive cathode, the electrolyte consists of HTeO.sub.2.sup.+ as the source of tellurium and Cd.sup.2+ as the source of cadmium. Discharged HTeO.sub.2.sup.+ ions at the cathode reacts with Cd.sup.2+ and form CdTe deposit on the cathode.
More specific conditions for CdTe electrodeposition and details of a process utilized to make thin film heterojunction solar cells using these films have been described in U.S. Pat. No. 4,388,483 granted to B. M. Basol, E. S. Tseng and R. L. Rod and assigned to Monosolar, Inc. Briefly in this patent, a sheet of an insulating transparent material, such as glass, is prepared with, on one side, a transparent conductive film, such as a tin oxide or indium tin oxide (ITO) layer, using conventional deposition techniques. Then a layer of a semiconductor, such as cadmium sulfide is electrodeposited. The combination of the conductive oxide and the cadmium sulfide comprise an n-type wide bandgap semiconductor different from the next layer deposited, which is cadmium telluride. This structure is then heat treated at a temperature between 250.degree. and 500.degree. C. for a time sufficient to convert the CdTe film to a substantially low resistivity p-type semiconductor compound. A conductive film, such as gold is then deposited on the cadmium telluride to complete the photovoltaic cell which receives radiation through the glass substrate and the n-type semiconductor acting as a wide bandgap window.
Heat treating the cadmium telluride was found to increase the power output of the photovoltaic cell by a factor of 60. It is believed that, in the absence of heat treatment, the electrodeposited cadmium telluride is a high resistivity n-type material and the cadmium sulfide serves as an electron injecting contact to one surface of the CdTe film rather than a rectifying contact. When the top conductor (e.g., gold) is deposited over the surface of the CdTe film, an n-CdTe/Au Schottky barrier is obtained. This is intrinsically a low efficiency structure. When heat treated (before depositing the Au), substantially all of the CdTe is converted to p type, due apparently to the generation of electrically active Cd vacancies. This shifts the barrier from the n-CdTe/Au interface to the CdS/p-CdTe interface and gives a high efficiency heterojunction structure.
Hg.sub.1-x Cd.sub.x Te is a very important infrared detector material. Its bandgap is a function of its stoichiometry and can be changed from 0 to 1.5 eV going from x=0.17 to x=1.0. So far the interest in this material has been limited to the infrared applications. Early work on Hg.sub.0.795 Cd.sub.0.205 Te detectors (sensitive at .lambda.=8-12 .mu.m) was later followed by investigation of structures that are suitable for use in the 1-3, 3-5, and 15-30 .mu.m range. All these applications require a Hg rich material (x&lt;0.5). A survey of previous literature shows no successful attempt of utilizing Cd rich (x&gt;0.5) mercury cadmium telluride for solar cell applications.
Hg.sub.1-x Cd.sub.x Te crystals can be prepared by techniques well known in the art (such as Bridgman growth, zone melting, and Czochralski). Epitaxial growth can be achieved by (liquid phase epitaxy LPE) and (vapor phase epitaxy VPE). There has not been much work on polycrystalline thin films of Hg.sub.1-x Cd.sub.x Te.
From this review of the prior art, it is apparent that there has been a failure to appreciate the potential of cadmium rich polycrystalline Hg.sub.1-x Cd.sub.x Te for solar cell applications. This may partly be due to the difficulties associated with the preparation of such films in an inexpensive way and with controlled stoichiometry.
Again the review of the prior art shows the lack of an inexpensive method for the production of Hg.sub.1-x Cd.sub.x Te films. The property of bandgap control for Hg.sub.1-x Cd.sub.x Te is extremely important for high efficiency stacked cells where two or more cells respond to different parts of the solar spectra. In the area of thin-film amorphous cells, there has been extensive research on variable bandgap alloys (such as amorphous Si-Ge alloys) that would be compatible with the top amorphous Si cell. But until this invention there has not been any success in finding a variable bandgap polycrystalline thin film that can be controllably and inexpensively produced and utilized.